Evaluates hypotheses that older persons have reduced capacity to use endogenous fat as fuel during exercise and that endurance training will improve their ability to use lipid as a fuel. Subjects will be healthy older volunteers and a group of healthy young volunteer controls. All subjects will undergo body composition, peak oxygen uptake, and lipid kinetic measurements (through plasma analysis of isotopic enrichments). Elderly subjects will then undergo exercise training program and repeat metabolism studies will be performed.